This invention relates to an exposure apparatus, and more particularly, to an exposure apparatus employing photolithiographic technology in the fabrication of semiconductor devices.
More specifically, this invention relates to a projection exposure apparatus having a reduction lens system devised to realize ultrafine processing in a photolithographic process used in the fabrication of semiconductor devices, and especailly to a reduction projection exposure apparatus using a discontinuous (for example, pulsive) energy beam, and even more specifically, to an exposure apparatus using an excimer laser.
At present, various reduction projection exposure apparatuses (steppers) are commercially available, using a superhigh pressure mercury lamp as the light source for facilitating ultrafine processing of semiconductor devices, especially LSI and VLSI. In the existing steppers, however, since the g-line (436 nm) and i-line (365 nm) of the light emitted by the superhigh pressure mercury lamp are used, the resolution is limited to 0.8 .mu.m for the g-line and to 0.6 .mu.m for the i-line. At these wavelengths, it is next to impossible to obtain a resolution of 0.5 .mu.m required in the manufacture of 4M and 16M bit devices.
Recently, in this area, it has been considered to modify the exposure apparatuses by using excimer laser light sources, such as XeCl (308 nm), KrF (249 nm) and ArF (193 nm), which emit light having a shorter wavelength than the g-line or i-line.
But the following problems are involved in the reduction projection exposure technology using an excimer laser.
(1) Because the exposure wavelength is short, the selection of material for optical material glass (vitreous material) used in the reduction projection lens is limited in relation to the transmission factor. For example, materials used in the lens for the laser wavelength of KrF (249 nm) and of ArF (193 nm) are limited to only SiO.sub.2 and CaF.sub.2. Therefore, with respect to the reduction projection lens design, it is required to correct chromatic aberration and to make other corrections in the unit in the order of one of several fractions of the wavelength by making use of the slight difference between the refractive index and dispersion of the glass material which will compose the lens. However, with the far ultraviolet rays of an excimer laser beam, since the usable materials are limited, it is only possible to shift radius of the curvature of the spherical material having the same refractive index by slight portions when designing of the reduction projection lens, which may result in an extreme length of about 1 meter only for the reduction projection lens.
If the reduction projection lens is this long, it is very difficult to reduce the quantity of light by shortening the optical path difference in the optical system to illuminate the reduction projection lens system and to design with an ample degree of freedom.
(2) On the other hand, reduction projection exposure apparatus not limited to the excimer laser, and installed in a superhigh performance clean room, its volume and height must be minimized due to the restrictions of the clean room. But if said lens is used in the general optical system of the reduction projection exposure apparatus, the arrangement shown in FIG. 5 will result. The light emitted from an excimer laser light source 1 passes through an integrator 2 and then from a flat plate refraction mirror 8 to an illumination optical system 7 having a vertical condenser lens for focusing and the light and then to a lens 5 over a distance l.sub.3 of nearly 1 meter, and the reticle 4 is uniformly illuminated, and the light is transmitted to the wafer semiconductor substrate 6 by way of a long reduction projection lens 5 having a lens length l.sub.1 of about 1 meter.
Therefore, in the conventional apparatus shown in FIG. 5, an illumination optical system 7 is required, and the length of l.sub.1 +l.sub.3 is as long as 2 meters, and the height of the apparatus itself is taller than 2 meters, so that the deviation of optical axis and other troubles are very likely to occur in the superhigh precision exposure apparatus forming patterns of about 0.5 .mu.m. And when the height is further increased, the design difficulties become more severe due to the problems of vibration and other problems, and this height problem is particularly serious in the excimer light source 1 which generates vibration noise upon every oscillation. Furthermore, it is extremely difficult to dispose the optical system of alignment line between the wafer 6 and the reticle 4. Finally, due to the extreme height of the apparatus, it is difficult to install the same in a superhigh performance clean room.
A sectional view of a conventional reduction projection exposure apparatus is shown in further detail in FIG. 6. An excimer laser light source 1 is secured to a stepper main body 12, and the laser beam emitted from the light source is reflected by a small reflection mirror 9, and is further reflected at a right angle by a large reflection mirror 8. In succession, the laser beam is focused by a condenser lens 7 onto the pupil of a lens 5, and an image is formed on a wafer 6 on an XYZ stage 6a through reticle 4 and lens 5. The main body is mounted on a quakeproof table 13. Numeral 11 designates power source and 10 designates a cable.